The purpose of the present study is to determine the safety and tolerability and potential clinical efficacy of subcutaneous doses of recombinant human IL-4 (rHuIL-4) in patients with rheumatoid arthritis (RA). RA is a chronic inflammatory disease of unknown etiology. It is characterized by synovial inflammation with frequent progression to articular cartilage and bone destruction. The synovial inflammatory response in RA consists predominately of T lymphocytes, mostly T helper cells (Th), with fewer numbers of macrophages and B lymphocytes. The Th cells can be divided into two overlapping subpopulations with important functional significance, Th1 and Th2 cells. Th1 cells secrete IL-2 and interferon- and regulate cell-mediated responses. Th2 cells secrete IL-4, IL-5, and IL-10, and mediate humoral responses. Most Th cells in the rheumatoid joint belong to the Th1 subpopulation. The synovial tissue in RA exhibits other features typical of a cell-mediated response, including hypervascularity, upregulation of cell adhesion molecules, and overexpression of pro-inflammatory cytokines such as IL-1 and tumor necrosis factor (TNF)-. IL-4, a cytokine produced by Th2 cells and macrophages, has primarily immunosuppressive and anti-inflammatory properties. The study is a multicenter, double-blind, placebo-controlled, clinical trial of IL-4 in escalating doses (0.5, 1.0, and 2.0 5g/kg) with a dosing period of 6 weeks. The main outcomes for the study are the frequency and severity of adverse effects and the American College of Rheumatology Core Disease Measures. The study has been completed. A total of four patients were enrolled at this site. The study population was predominantly female (3/1) and Caucasian. The study drug was well tolerated and did not cause any serious adverse event reactions.